The present invention relates to a wireless network system, and more particularly to a technique of a multihop wireless communication system.
Coping with increasing demands for natural resources becomes an international task. In the field of resources surveys, necessity to reasonably provide survey sensor techniques of high sensitivity is increasing. Resource survey systems are acceleratingly shifted to large scale systems. The resource survey system has an increasing importance to measure subsurface structures in high resolution by densely arranging sensors.
For example, as one of primary methods used for identifying deposit sites of resources in resources surveys is a method referred to as reflection seismology. In this method, controlled seismic sources arranged on the ground surface generate artificial vibrations, and the vibrations are reflected and returned from layers under the ground, such as a soil layer, a water layer, layers that reserve petroleum and gases, and a basement rock layer. The waves of the returned vibrations are received by vibration sensors (acceleration sensors) arranged on the ground surface. The signal waveforms of the waves are analyzed to grasp stratum structures and crustal structures.
For the controlled seismic source, a special vehicle referred to as an earthquake simulation vehicle, which can artificially generate vibrations, is sometimes used. In the case of reflection seismology, an earthquake simulation vehicle that is a controlled seismic source, sensors to detect vibrations, a sensor network system that transfers acquired vibration data (in the following, referred to as “a sensor network”), and a data center to accumulate the acquired vibrations data (a data collection vehicle) are necessary in the research subject field.
However, a wired connection configuration has a limitation on the number of sensors that can provide simultaneous measurement. The wired connection configuration has restrictions on its installation design depending on obstructions, such as trees, rocks, and revers, which exist in research subject fields including forests and dense forests. The wired connection configuration needs field facilities, such as a large-capacity establishment to supply power to vibration sensors and a large-scale data center (a data collection vehicle). These contribute to cost increases in the sensor network. Therefore, a system configuration that omits wired connection while sensors are densely arranged is desired.
International Publication No. WO2006/090480 describes a network management method using small-sized wireless sensor nodes in which a base station and sensor nodes have counter values to be the references to the operation interval and the operation start phase of the sensor nodes, and the counter values are synchronized at regular time intervals.